Stress Localization and Kinking as a Potential Source of Rheological Weakening in the High-Stress Deformation of Polycrystalline Ice

Abstract:

Constraining water ice rheology is crucial for geodynamic modelling of terrestrial ice masses and to understand the mechanics of icy planets in the outer solar system. Creep experiments on homogenous laboratory-grown ice have been conducted for decades with the goal to link specific stress and temperature conditions to creep (strain) rates, which are governed by the operating microstructural deformation mechanism. As most of these experiments have been conducted under constant strain rate conditions and in the absence of a time-effective method to image fine-grained ice, the response of an ice microstructure to a constant stress experiment is fairly unknown. In this study, 25 mm diameter cylinders of polycrystalline ice with a starting average grain diameter of 400 µm were subjected to a confining pressure of 50 MPa and axial loads between 3 and 13 MPa at a temperature of 240 K. The samples were subsequently imaged with cryogenic electron backscatter diffraction (cryo-EBSD). Over the entire range of these tests, constantly accelerating strain rates were observed, which represent a significant rheological weakening. Microstructural maps of the deformed ice samples show stress localizations that are characterized by excessive kinking, leading to a “crushed” appearance of pre-existing grains. The localized kinking can produce grain diameters as small as 30 µm and yields a local grain size reduction that could provide an explanation for the rheological weakening, as observed in the accelerating strain rates. A detailed microstructural analysis aims to investigate the mechanism of kinking in these stress localizations with a microstructural misorientation analysis comprising both pre-existing and kinked grains. Grain size data collected from within the high-stress regions will be compared to pre-existing rheological data to assess if the localized grain size reduction could in fact result in the observed accelerations in strain rate.